1. Field of the Invention
The present invention relates to an image forming apparatus for forming a visible image by superimposing a plurality of color images on top of one another, the image forming apparatus having a function of correcting misalignment of image positions of the plurality of color images, and a method of correcting image misalignment.
2. Description of the Background Art
Typically, image forming apparatuses employing electrophotography form a full-color visible image by superimposing a plurality of color images on top of each other. For example, image forming apparatuses may use four single colors for image forming, in which a single image is formed with each of the four colors, and then four single-color images are superimposed to form a full-color image. Such image forming apparatuses may be known as tandem-type image forming apparatuses, for example.
The tandem-type image forming apparatus typically employs either an indirect transfer system or a direct transfer system. In the indirect transfer system, an image formed on an image bearing member is initially transferred onto an intermediate transfer belt, whereas in the direct transfer system, an image formed on an image bearing member is directly transferred onto a transfer sheet transported on a sheet transport belt.
In such apparatuses, a color pattern is used to detect and correct misalignment between images. Accordingly, a color pattern used for correcting image misalignment between images may be formed for each color on the intermediate transfer belt in the indirect transfer system but on the sheet transport belt in the direct transfer system. Such correction-use patterns may be detected by an optical sensor, such as a toner marking (TM) sensor, to correct an image write-timing so that four single-color images can be superimposed correctly to form a single full-color image. Such tandem-type image forming apparatus is disclosed in JP-2858735-B and JP-2642351-B, for example.
With the use of such optical sensors, the spectral sensitivity of the optical sensors becomes an important consideration. For example, JP-2007-240591-A discloses a light scanning unit including a light source such as a laser diode (LD), an optical system, and at least one detector such as a photodiode, which can maintain a stable output signal even when certain properties of the laser diodes vary among different manufacturing lots or when the use environment of the light scanning unit changes. The optical system deflects a light beam emitted from the light source to scan an image bearing member and the detector detects the light beam at a given position. In such light scanning unit, the laser diode used as the light source has an oscillation wavelength shorter than 450 nm, and the optical system includes an optical member having a spectral sensitivity that is the opposite of the spectral sensitivity of the photodiode used as the detector.
Further, JP-2004-21164-A discloses a color image forming apparatus including an image concentration sensor to detect concentration of images. The image concentration sensor includes a light source, which emits visible light toward a target image, and a light-receiving sensor, which detects light reflected from the target image. In such image forming apparatus, a light source suitable for the detection process employed is provided for each color. Accordingly, the number of image concentration sensors must match the number of colors, thus increasing the overall cost of the color image forming apparatus.
In light of the above-described situation, there has been proposed an image forming apparatus including an image concentration sensor to detect concentration of images, in which a light source emits visible light toward a target image and light reflected from the target image is detected to determine the image concentration. In such image forming apparatus, there are fewer light sources than colors to be detected, and a single light-receiving sensor is used in common for all colors to provide good detection precision at reduced cost.
A TM sensor to detect the correction-use pattern may include a light-emitting diode (LED) as a light emitting device and a photodiode (PD). The LED directs a beam of light onto either a sheet transport belt or an intermediate transfer belt and the PD receives light reflected from the belt. Such reflected light includes a regular reflected light component and a diffuse reflected light component. The TM sensor uses the regular reflected light component to detect the correction-use pattern because the regular reflected light is reflected from a surface of the belt strongly but not reflected from a toner image, whereas the diffuse reflected light is reflected from a toner image of the color pattern (not including black) weakly but not reflected from a surface of sheet transport belt and a black toner image.
As such, in a process of correcting image misalignment, the diffuse reflected light component signal may not be needed. Accordingly, the TM sensor may employ a configuration to remove the diffuse reflected light component before the reflected light enters a light receiving unit such as a PD. In this case, a slit or a focus lens may be used to remove the diffuse reflected light component and the PD is used as a light receiving unit to receive a regular reflected light component. However, such configuration may increase the cost of the TM sensor. By contrast, in a lower-cost TM sensor, which does not need such configuration, a light receiving unit such as the PD receives the regular reflected light and the diffuse reflected light mixed together when detecting a correction-use pattern.
However, if the LED and the PD are out of alignment due to mechanical tolerance or assembly error, a color pattern detection signal may include both the regular reflected light component and the diffuse reflected light component, in which a peak position of the regular reflected light component and a peak position of diffuse reflected light component do not match. Such unmatched peak position condition may result in image misalignment detection error.